Personal computing devices such as portable wireless telephones and personal digital assistants (PDAs) are requiring ever-increasing data storage capacity to perform a continuously longer list of applications. For example, a wireless telephone can include a digital video camera, video and audio file player, portable game player, and Internet access/web browser. Concurrently though, portable communication devices are becoming ever smaller and, since the processing for such tasks can consume considerable power, and battery life is highly valued, power consumption by the data storage is preferably kept to a minimum.
Resistance-memories, which store data as a switchable resistance, show promise in meeting currently anticipated data storage needs of personal computing devices, as they may provide high capacity, show increasing access speeds, and have low power consumption.
One kind of resistance memory device is the magnetic random access memory (MRAM). MRAMs typically consist of a row-column array of magnetic tunneling junction (MTJ) transistors. Data is stored in the MTJ by selective current flow to induce a magnetic field, switching the MTJ's magnetization between two states. The resistance of the MTJ corresponds to its magnetic state and is readable. Reading may be performed by injecting a data read current through the storage MTJ to cause a read voltage and, concurrently, injecting a reference read current of approximately the same value through reference MTJs having a pre-set reference resistance state. The pre-set reference resistance states are selected such that the reference voltage is halfway between the storage MTJ's read voltage at its high resistance state and low resistance state. Therefore, comparing the read voltage against the reference voltage indicates, preferably with acceptable accuracy, the storage MTJ's resistance state.